On Monday, Microsoft pushed out an emergency out-of-band update for a critical security vulnerability that could allow an attacker to gain complete control of your system. The flaw affects all systems from Windows Vista and up--including the Windows 10 Technical Preview.
The exploit works if you open a document or visit a webpage that contains malicious embedded OpenType fonts. Those bad-acting fonts then take advantage of a weakness in how the Windows Adobe Type Manager Library handles OpenType fonts. OpenType is a font file format developed by Adobe and Microsoft.
The security update is available immediately through the Windows Update mechanism. If you're set up for automatic updates, your PC should download the update later today, if it hasn't already. Anyone doing manual updates should check Windows Update immediately.
To verify if your PC has the critical update on Windows 8.1 (Windows Vista and 7 users will have a similar process), open the Control Panel, search for "Windows Update", and then choose View update history. Towards the top of the list you should see a security update with the number KB3079904.
If you don't see it, and want to install this update right away, run Windows Update manually--a restart will be required.
This latest security flaw is yet another revelation from the trove of documents published from the Hacking Team breach. Prior to the OpenType flaw, Adobe was kept busy patching several vulnerabilities in Flash that prompted Mozilla to disable the Flash Player browser plugin in Firefox.
The impact on you at home: Considering this is a critical security update that puts your whole system at risk, it's worth installing this update as soon as you can. If you can't pause for a reboot right now, be extra wary of any documents you download to your system today and stick to mainstream websites. That's not a perfect solution, but for most it should be good enough until you have time to get the update or wait for the automated process to kick in.